This invention relates to logging of earth boreholes to obtain fractional flow characteristics of formations that are indicative of formation producibility and, more particularly, to a logging technique which utilizes resistivity measurements in the obtainment of fractional flow characteristics.
The logging of formations surrounding earth boreholes to obtain indications of the general nature of the fluids present in rock, and to obtain indications of the porosity of the rock, is a well developed art. For example, resistivity logs, which can measure formation resistivity at a specified radial depth of investigation into the formations, can be used to distinguish between oil bearing formations and water bearing formations. [The term "oil" is used herein for ease of description, and can mean other hydrocarbons as well.] As a simple example, if it is generally known that the connate water in formations uninvaded by drilling filtrate is relatively salty (and therefore of relatively low resistivity), a resistivity measurement that "sees" deep into the formation will measure a relatively low resistivity in a water bearing formation and a relatively high resistivity in an oil bearing formation. The resistivity of the so-called invaded (or "flushed") zone adjacent the borehole, where drilling fluid filtrate has replaced at least some of the original fluids in the invaded formations, is typically measured with a resistivity logging device having a relatively shallow radial depth of investigation, and can also be very useful in determining formation properties. The porosity of formations is also determinable from well logging devices such as acoustic logging tools or neutron logging tools, and provides a useful measure of the pore space volume of formations. Also, the conductivity of the connate water in the pore spaces of uninvaded formations can be determined by various means including, for example, logging measurements from a spontaneous potential ("SP") logging tool, or sampling with a formation testing device capable of hydraulically contracting the borehole wall and drawing fluid samples from the formations.
If one has reasonably accurate knowledge of the types of characteristics described in the previous paragraph (formation resistivity, porosity, and conductivity of water contained in the formations), one can derive an indication of water saturation in the formation pore spaces using the well known Archie saturation equation. Water saturation, S.sub.w, which is the complement of oil saturation, S.sub.o, can be an important indication of a volume of oil in a given formation, but it is well recognized that the producibility of a formation also depends strongly on the degree to which oil in the formation can move through the rock for ultimate recovery.
Various factors control the degree to which oil and water will move through rock including, inter alia, the tortuosity of travel paths and the extent to which oil and water are bound to the rock by mobility restricting forces. The fractional flow characteristics of formations are generally representable by the fractional flow curve for such formations, which describes the flow of water (and, therefore, by complementary relationship, oil) in relation to total flow, as a function of water saturation. Important fractional flow characteristics that define the fractional flow curve include: S.sub.wr (the residual water saturation), S.sub.or (the residual oil saturation), S.sub.orm (the maximum possible residual oil saturation, which is related to S.sub.or), and S.sub.wc (the connate water saturation). The fractional flow at connate water saturation, S.sub.wc, is designated f.sub.wc.
A reasonable determination of fractional flow characteristics, including those just listed, would be very helpful to production engineers in predicting the production performance of formations, since these characteristics contain information about the fractions of oil and water that may or may not be moved, as well as the flow behavior that can be expected for the fluids that do move. However, in the prior art, some or all of these characteristics have either not been obtainable with sufficient accuracy by any means, or have required formation tests and procedures (including tests which perforate, sample, and/or core) that are time consuming and expensive, and may not be practical to perform for more than a few selected zones along the length of the borehole.
It is among the objects of the present invention to provide a technique for determining fractional flow characteristics of formations surrounding a borehole, and generating improved logs of said characteristics, without requirement for unduly time consuming and/or expensive downhole procedures and techniques.